Scanning optical system

ABSTRACT

A scanning optical system has an object side lens unit for condensing light from an object, a mirror for performing a main scan to read the image of the object by deflecting the light transmitted through the object side lens and an image side lens unit for forming an image on the image sensing surface using both the extra-axial light and axial light in a subscan direction deflected by the mirror. The scanning optical system satisfies the predetermined condition defined by lenses of Abbe number.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Patent Application No. 11-72910 filed in Japan, the content of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a scanning optical system, for example a scanning optical system used in a film scanner or the like capable of high-speed image retrieval.

[0004] 2. Description of the Related Art

[0005] Various scanning optical systems have been proposed for use in film scanners and the like. Among the proposed systems for high-speed, high-precision image retrieval is a scanning optical system for forming a film image on a unidimensional linear image sensing element (e.g., a line CCD) having an array of photoreceptor elements arranged in a subscan direction via a mirror rotated in a main scan direction to read an object, scanning optical systems suitable for this type of mirror scanning optical system have been disclosed in Japanese Laid-Open Patent Application Nos. 9-236741, 9-236747, 9-236766, and 9-236767.

[0006] The scanning optical systems disclosed in these patents use a color separation prism to accomplish so-called 3-panel color separation of color images, and therefore are disadvantageously expensive. Conventionally, in order to reduce cost while maintaining high resolution, it has been thought most effective to eliminate the color separation prism while using a tri-linear image sensing element having the aforesaid unidimensional line image sensing element arrayed in three lines in the main scan direction.

[0007] When the aforesaid tri-linear image sensing element is used, however, it becomes necessary not only to correct magnification chromatic aberration, but also axial chromatic aberration which is not a problem in optical systems using the color separation prism. That is, identical focusing is required on the three line image sensing elements on the same surface. When scanning via a mirror, severe disadvantages arise in the scanning optical system from the perspective of the various types of aberration generated, e.g., chromatic aberration, coma and the like.

SUMMARY OF THE INVENTION

[0008] In view of the previously mentioned disadvantages, an object of the present invention is to provide a high performance scanning optical system at low cost which specifically corrects magnification chromatic aberration and axial chromatic aberration without using a color separation prism, and is not susceptible to performance deterioration when scanning via a mirror.

[0009] These objects are attained by a scanning optical system having, an object side lens unit for condensing light from an object, a mirror for performing a main scan to read the image of the object by deflecting the light transmitted through the object side lens; and an image side lens unit for forming an image on the image sensing surface using both the extra-axial light and axial light in a subscan direction deflected by the mirror; and wherein the following condition is satisfied.

[0010] 0.1<|(Σνfp−Σνfm)/Lf|<20.0

[0011] Where Σνfp represents the sum of the Abbe numbers of the positive optical power lenses within the object side lens unit, Σνfm represents the sum of the Abbe numbers of the negative optical power lenses within the object side lens unit, and Lf represents the number of lenses in the object side lens unit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] These and other objects and features of this invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanying drawings, in which:

[0013]FIG. 1 is a constructions of the scanning optical systems of the first embodiment (the angle range of θ=45°);

[0014]FIG. 2 is a constructions of the scanning optical systems of the first embodiment (the angle range of θ=45°−6.1°);

[0015]FIG. 1 is a constructions of the scanning optical systems of the first embodiment (the angle range of θ=45°);

[0016]FIG. 2 is a constructions of the scanning optical systems of the first embodiment (the angle range of θ=45°−6.1°);

[0017]FIG. 1 is a constructions of the scanning optical systems of the first embodiment (the angle θ=45°);

[0018]FIG. 2 is a constructions of the scanning optical systems of the first embodiment (the angle θ=45°−6.1°);

[0019]FIG. 3 is a constructions of the scanning optical systems of the second embodiment (the angle θ=45°);

[0020]FIG. 4 is a constructions of the scanning optical systems of the second embodiment (the angle θ=45°−6.1°);

[0021]FIG. 5 is a constructions of the scanning optical systems of the third embodiment (the angle θ=45°);

[0022]FIG. 6 is a constructions of the scanning optical systems of the third embodiment (the angle θ=45°−6.1°):

[0023]FIG. 7 is a constructions of the scanning optical systems of the fourth embodiment (the angle θ=45°);

[0024]FIG. 8 is a constructions of the scanning optical systems of the fourth embodiment (the angle θ=45°−6.1°);

[0025]FIG. 9 is a constructions of the scanning optical systems of the fifth embodiment (the angle θ=45°);

[0026]FIG. 10 is a constructions of the scanning optical systems of the fifth embodiment (the angle θ=45°−6.1°);

[0027]FIG. 11 is an aberration diagram of the first embodiment, FIG. 11A shows spherical aberration diagram, FIG. 11B shows astigmatism and FIG. 11C shows distortion;

[0028]FIG. 12 is an aberration diagram of the second embodiment, FIG. 12A shows spherical aberration diagram,. FIG. 12B shows astigmatism and FIG. 12C shows distortion.

[0029]FIG. 13 is an aberration diagram of the third embodiment, FIG. 13A shows spherical aberration diagram, FIG. 13B shows astigmatism and FIG. 13C shows distortion;

[0030]FIG. 14 is an aberration diagram of the fourth embodiment, FIG. 14A shows spherical aberration diagram, FIG. 14B shows astigmatism and FIG. 14C shows distortion; and

[0031]FIG. 15 is an aberration diagram of the fifth embodiment, FIG. 15A shows spherical aberration diagram, FIG. 15B shows astigmatism and FIG. 15C shows distortion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] The preferred embodiments of the present invention are described hereinafter with reference to the accompanying drawings. FIGS. 1 and 2, FIGS. 3 and 4, FIGS. 5 and 6, FIGS. 7 and 8, FIGS. 9 and 10 respectively show the constructions of the scanning optical systems of the first through fifth embodiments. In the drawings, the x-axis, y-axis, and z-axis are shown mutually perpendicular one to another: the drawing surface is designated the x-y plane, and the z-axis is perpendicular to the drawing surface toward the foot of the drawing. These scanning optical systems are of a mirror scanning type comprising sequentially from the image side an image side lens unit Gr1, stop S, mirror M, and object side lens unit Gr2.

[0033] A film screen not shown in the drawing is arranged as an object at a stationary position within the image sensing range on the object side of the scanning optical system, (i.e., the left side of lens unit Gr2 in each drawing), and a tri-linear image sensing element 1 is arranged as an image sensing surface on the image side of the scanning optical system, (i.e., the bottom side of the lens unit Gr1 in each drawing). The tri-linear image sensing element 1 is disposed with the z-axis direction (subscan direction) designated as the lengthwise direction, and the unidimensional line image sensing elements are arranged in three lines in the x-axis direction (main scan direction) (details not shown in the drawings).

[0034] The light from the film screen condensed by the object side lens unit Gr2 is deflected by the mirror M, and the axial light and extra-axial light in the subscan direction both form an image on the tri-linear image forming element 1 via the image side lens unit Gr1. The image side lens unit Gr1 and the object side lens unit Gr2 comprise rotationally symmetrical spherical surface lenses, and have a symmetrical type construction advantageous in aberration correction via the mirror M.

[0035]FIGS. 1, 3, 5, 7, and 9 show the optical path L at a rotational angle relative to the Y-axis of the mirror M, i.e., the mirror oscillation angle, of θ=45°. The angle range of θ=45°±6.1°centered on the mirror oscillation angle of θ=45°becomes the main scanning range. FIGS. 2, 4, 6, 8, and 10 show the optical path La at a mirror oscillation angle of θ=45°−6.1°. The angle range of θ=45°±6.1°centered on the mirror oscillation angle of θ=45° becomes the main scanning range. Although the optical path has been omitted at θ=45°+6.1°, the optical path through the object side lens unit Gr2 at this time is approximately symmetrical to the optical axis X in FIGS. 1, 3, 5, 7, 9 relative to the optical path shown in FIGS. 2, 4, 6, 8, 10.

[0036] In the first embodiment shown in FIGS. 1 and 2, the object side lens unit Gr2 comprises sequentially from the object side ten lens elements G1˜G10 along the optical axis X, and the image side lens unit Gr1 comprises sequentially from the object side six lens elements G11˜G16 along the optical axis Y. The object side lens unit Gr2 can be understood as comprising sequentially from the object side an object side front lens unit Gr2-1 and an object side back lens unit Gr2-2 arranged with a relatively large spacing therebetween. The object side front lens unit Gr2-1 is a cemented lens comprising two lens elements G1 and G2, and the object side back lens unit Gr2-2 comprises eight lens elements G3˜G10.

[0037] In the second embodiment shown in FIGS. 3 and 4, the object side lens unit Gr2 comprises sequentially from the object side nine lens elements G1˜G9 along the optical axis X, and the image side lens unit Gr1 comprises sequentially from the object side six lens elements G10-G15 along the optical axis Y. The object side lens unit Gr2 can be understood as comprising sequentially from the object side an object side front lens unit Gr2-1 and an object side back lens unit Gr2-2 arranged with a relatively large spacing therebetween. The object side front lens unit Gr2-1 is a cemented lens comprising two lens elements G1 and G2, and the object side back lens unit Gr2-2 comprises seven lens elements G3˜G9.

[0038] In the third embodiment shown in FIGS. 5 and 6, the object side lens unit Gr2 comprises sequentially from the object side nine lens elements G1˜G9 along the optical axis X, and the image side lens unit Gr1 comprises sequentially from the object side six lens elements G10˜G15 along the optical axis Y. The object side lens unit Gr2 can be understood as comprising sequentially from the object side an object side front lens unit Gr2-1 and an object side back lens unit Gr2-2 arranged with a relatively large spacing therebetween. The object side front lens unit Gr2-1 is a cemented lens comprising two lens elements G1 and G2, and the object side back lens unit Gr2-2 comprises seven lens elements G3˜G9.

[0039] In the fourth embodiment shown in FIGS. 7 and 8, the object side lens unit Gr2 comprises sequentially from the object side eight lens elements G1˜G8 along the optical axis X, and the image side lens unit Gr1 comprises sequentially from the object side six lens elements G9˜G14 along the optical axis Y. The object side lens unit Gr2 can be understood as comprising sequentially from the object side an object side front lens unit Gr2-1 and an object side back lens unit Gr2-2 arranged with a relatively large spacing therebetween. The object side front lens unit Gr2-1 is a cemented lens comprising two lens elements G1 and G2, and the object side back lens unit Gr2-2 comprises six lens elements G3˜G8.

[0040] In the fifth embodiment shown in FIGS. 9 and 10, the object side lens unit Gr2 comprises sequentially from the object side seven lens elements G1˜G7 along the optical axis X, and the image side lens unit Gr1 comprises sequentially from the object side five lens elements G8˜G12 along the optical axis Y. The object side lens unit Gr2 can be understood as comprising sequentially from the object side an object side front lens unit Gr2-1 and an object side back lens unit Gr2-2 arranged with a relatively large spacing therebetween. The object side front lens unit Gr2-1 is a cemented lens comprising two lens elements G1 and G2, and the object side back lens unit Gr2-2 comprises five lens elements G3˜G7.

[0041] Desirable conditions for the scanning optical system are described below. In scanning optical systems comprising an object side lens unit for condensing light from an object, a mirror for performing a main scan to read the image of the object by deflecting the light transmitted through the object side lens, and an image side lens unit for forming an image on the image sensing surface using both the extra-axial light and axial light in a subscan direction deflected by the mirror, such as in the previously described embodiments, magnification chromatic aberration is generated by mirror scanning. In order to reduce this chromatic aberration, the chromatic aberration of each lens unit must be preset at a desirable state. Furthermore, it is desirable to accomplish aberration correction by negating each aberration via the respective lens units by correcting the aberration generated by the object side lens unit via the image side lens unit.

[0042] In the scanning optical systems of the above mentioned embodiments, it is desirable that the object side lens unit satisfies conditional equation (1) below. When this condition is satisfied, excellent chromatic aberration correction can be achieved, and a high-performance optical system can be realized.

0.1<|(Σνfp−Σνfm)/Lf|<20.0  (1)

[0043] Where Σνfp represents the sum of the Abbe numbers of the positive optical power lenses within the object side lens unit, Σνfm represents the sum of the Abbe numbers of the negative optical power lenses within the object side lens unit, and Lf represents the number of lenses in the object side lens unit.

[0044] Chromatic aberration generated within the object side lens unit can be reduced, and particularly a reduction of axial chromatic aberration is achieved in the complete optical system by satisfying the above conditions. When the upper limit of condition (1) is exceeded, chromatic aberration generated by the positive optical power lens elements within the object side lens unit is particularly increased, so as to make it difficult to correct the chromatic aberration generated by the object side lens unit via the image side lens unit. Conversely, when the lower limit of condition (1) is exceeded, chromatic aberration generated by the negative optical power lens elements within the object side lens unit is particularly increased, so as to make it difficult to correct the chromatic aberration generated by the object side lens unit via the image side lens unit.

[0045] In the scanning optical systems of each of the previously mentioned embodiments, it is desirable that the image side lens unit satisfies conditional equation (2) below.

10.0<|(Σνrp−Σνrm)/Lr|<50.0  (2)

[0046] Where Σνrp represents the sum of the Abbe numbers of the positive optical power lenses within the image side lens unit, Σνrm represents the sum of the Abbe numbers of the negative optical power lenses within the image side lens unit, and Lr represents the number of lenses in the image side lens unit.

[0047] Chromatic aberration generated by the object side lens unit can be corrected by the image side lens unit, and a high-performance optical system can be realized by satisfying the above condition. When the upper limit of condition (2) is exceeded, chromatic aberration generated by the positive optical power lens elements within the image side lens unit is particularly increased, so as to make it difficult to correct the chromatic aberration generated by the object side lens unit via the image side lens unit. Conversely, when the lower limit of condition (2) is exceeded, chromatic aberration generated by the negative optical power lens elements within the image side lens unit is particularly increased, so as to make it difficult to correct the chromatic aberration generated by the object side lens unit via the image side lens unit.

[0048] Although it is desirable to correct the chromatic aberration of the total optical system by correcting the chromatic aberration generated by the object side lens unit via the image side lens unit, if the chromatic aberration of each lens unit is becomes too large, an undesirable effect is magnification chromatic aberration becomes particularly large when scanning via a mirror. In correspondence therewith, the previously mentioned conditions aim not only to reduce axial chromatic aberration, but also to reduce magnification chromatic aberration generated during mirror scanning.

[0049] In the scanning optical systems of each of the previously mentioned embodiments, it is desirable that the cemented lens comprises a negative lens element and a positive lens element on the outermost object side. In this way, it is possible to achieve excellent correction of magnification chromatic aberration, particularly extra-axial magnification chromatic aberration. It is further desirable that the cemented lens on the outermost object side within the object side lens unit satisfies conditional equation (3) below.

|φf−1/φf|<1.0  (3)

[0050] Where φf−1 represents the optical power of the cemented lens on the outermost object side within the object side lens unit, and φf represents the optical power of the object side lens unit.

[0051] Condition (3) is a condition for correcting chromatic aberration, and particularly extra-axial aberration, via the cemented lens on the outermost object side within the object side lens unit. When the upper limit of condition (3) is exceeded, it becomes difficult to correct the various types of aberration such as magnification chromatic aberration and extra-axial aberration generated by the lens elements.

[0052] In the scanning optical systems of each of the aforementioned embodiments, it is desirable that the object side lens unit comprises sequentially from the object side an object side front lens unit and an object side back lens unit disposed with a relatively large space therebetween. In this way, a difference is created in the ray height of the paraxial marginal ray between the outermost image side lens element of the object side front lens unit and the outermost object side lens element of the object side back lens unit, which is greatly effective in correcting the extra-axial aberration in particular.

[0053] It is desirable that the outermost image side lens element within the object side front lens unit and the outermost object side lens element within the object side back lens unit satisfy conditional equation (4) below.

0.4<Rf−1r/Rf−2f<5.0  (4)

[0054] Where Rf−1r represents the radius of curvature of the image side surface of the outermost image side lens element within the object side front lens unit, and Rf−2f represents the radius of curvature of the object side surface of the outermost object side lens element within the object side back lens unit.

[0055] Condition (4) is a condition for correcting various types of aberration, most importantly spherical aberration and extra-axial coma generated within the lens unit of the object side lens unit, and is a condition particularly for balancing the aberration generated by over-correction by the object side front lens unit by means of the image side lens element. When this condition is eliminated, the balance of aberration correction is adversely affected within the object side lens element, resulting in insufficient correction or over-correction of aberration, and undesirably generating high order aberration in particular.

[0056] It is desirable that the object side lens unit satisfies conditional equation (5) below.

0.05<Tf−12×φf<0.4  (5)

[0057] Where Tf−12 represents the distance between the object side front lens unit and the object side back lens unit, and φf represents the optical power of the object side lens unit.

[0058] Condition (5) relates to the spacing between the object side front lens unit and the object side back lens unit, and from aberration theory is greatly effective in correcting the extra-axial aberration in particular by creating a difference in the ray height of the paraxial marginal ray between the object side front lens unit and the object side back lens unit. When the lower limit of condition (5) is exceeded, sufficient difference in ray height of the paraxial marginal ray cannot be obtained. Conversely, when the upper limit is exceeded, the total length of the object side lens unit is increased, so as to undesirably increase the space required by the optical system.

[0059] In the scanning optical system of the previously mentioned embodiments, it is desirable that the exit pupil of the object side lens unit approximately matches the entrance pupil of the image side lens unit. For this reason, it is desirable to provide a common stop for both the object side lens unit and the image side lens unit near the mirror used for scanning.

[0060] The object side lens unit desirably possesses at least one negative lens element for chromatic aberration correction, spherical aberration correction, and Petzval sum correction. In this way, the height of the pass-through ray is increased so as to effectively correct spherical aberration. Furthermore, the height of the extra-axial light passing through the lens is also increased, which is advantageous in correcting extra-axial aberration.

[0061] It is desirable that the extra-axial luminous flux heightened at the lens passage position nearly matches the passage position near the stop position, and to achieve this end, it is desirable that the outermost image side lens element within the object side lens unit has a concave surface on the image side, and it is particularly desirable that this lens element having a concave meniscus shape on the image side. In this way, extra-axial aperture efficiency is maintained, and mirror used for the main scan can be made more compact.

[0062] It is desirable that the negative lens of the image side lens unit satisfies conditional equation (6) below.

νrm<35.0  (6)

[0063] Where νrm represents the Abbe number of the negative lens in the image side lens unit. Condition (6) is a condition for obtaining a desired chromatic aberration generated within the image side lens unit. When the range of condition (6) is omitted, it becomes difficult to correct the aberration generated by the positive lens element within the image side lens unit via the negative lens element.

[0064] When considering aberration correction of the entire optical system, it is necessary to balance the respective aberrations generated by the object side lens unit and the image side lens unit. Although it is desirable that the negative lens element in the object side lens unit have a relatively large optical power to correct chromatic aberration, spherical aberration and extra-axial aberration, the large optical power generates negative aberration. In order to correct this aberration in the negative direction, it is desirable that the surface of the outermost object side lens element within the image side lens unit is a positive lens element having a convex shape on the object side. In this way, the pass-through position of the extra-axial luminous flux from the object side lens unit can be reduced in the lens element following the image side lens unit, so as to have a more compact lens diameter.

[0065] It is further desirable that the surface of the lens element on the outermost image side in the image side lens unit is a concave surface on the image side. In this way, extra-axial aberration, and particularly distortion, can be corrected in a desired direction.

[0066] The construction of the photographic optical system of the present invention is described below by way of specific examples of construction data and aberration diagrams. The optical systems of examples 1˜5 respectively correspond to the optical systems of the first through fifth embodiments. The structural diagrams (FIGS. 1 and 2, FIGS. 3 and 4, FIGS. 5 and 6, FIGS. 7 and 8, FIGS. 9 and 10) representing the scanning optical systems of the first through fifth embodiments respectively show the construction of the corresponding optical systems of examples 1˜5.

[0067] In each example, ri (i=1,2,3 . . . ) represents the No. i surface and the radius of curvature of the No. i surface counting from the object side, di (i=1,2,3 . . . ) represents the axial distance of the No. i surface counting from the object side, Gi (i=1,2,3 . . . ) represents the No. i lens element counting from the object side, Ni (i=1,2,3) and vi (i=1,2,3 . . . ) respectively represent the d-line refractive index and Abbe number of the No. i lens element counting from the object side. Embodiment 1 Object distance: 60 mm Effective F. No.: 5.0 Mirror oscillation angle: ±6.1 degrees [Radius of [Axial [Refractive [Abbe Curvature] Distance] [Lens] Index] Number]  r1 =    91.255  d1 =  9.00  G1  N1 = 1.75450  ν1 = 51.57  r2 =   197.911  d2 =  2.50  G2  N2 = 1.67339  ν2 = 29.25  r3 =    49.624  d3 = 22.89  r4 =    73.094  d4 = 12.00  G3  N3 = 1.61800  ν3 = 63.39  r5 =  −56.042  d5 =  1.50  r6 =  −53.916  d6 =  4.00  G4  N4 = 1.67339  ν4 = 29.25  r7 =    43.252  d7 =  2.00  r8 =    52.205  d8 = 10.49  G5  N5 = 1.83350  ν5 = 21.00  r9 =  −59.216  d9 =  1.00 r10 =  −59.339 d10 =  2.50  G6  N6 = 1.67339  ν6 = 29.25 r11 =   295.777 d11 =  1.00 r12 =    33.245 d12 =  9.85  G7  N7 = 1.61800  ν7 = 63.39 r13 = −131.394 d13 =  2.00 r14 = −116.242 d14 =  2.50  G8  N8 = 1.74000  ν8 = 31.72 r15 =    28.824 d15 =  2.00 r16 =    27.762 d16 =  4.76  G9  N9 = 1.78831  ν9 = 47.32 r17 =   110.880 d17 =  1.00 r18 =    24.041 d18 =  2.00 G10 N10 = 1.74000 ν10 = 31.72 r19 =    16.756 d19 = 30.00 r20 = ∞ d20 = 13.00 (Mirror M) r21 = ∞ d21 =  4.50 (Stop S) r22 =    21.064 d22 =  2.50 G11 N11 = 1.83350 ν11 = 21.00 r23 =    30.406 d23 =  5.68 r24 =   103.496 d24 =  3.00 G12 N12 = 1.61800 ν12 = 63.39 r25 =  −21.355 d25 =  1.50 r26 =  −17.416 d26 =  4.00 G13 N13 = 1.67339 ν13 = 29.25 r27 =    23.035 d27 =  2.00 r28 =    17.575 d28 =  6.00 G14 N14 = 1.48749 ν14 = 70.44 r29 =    17.480 d29 =  1.91 r30 =    38.082 d30 =  6.00 G15 N15 = 1.61800 ν15 = 63.39 r31 =  −42.455 d31 = 14.74 r32 =    18.131 d32 =  6.00 G16 N16 = 1.83350 ν16 = 21.00 r33 =    15.603

[0068] Embodiment 2 Object distance: 60 mm Effective F. No.: 5.0 Mirror oscillation angle: ±6.1 degrees [Radius of [Axial [Refractive [Abbe Curvature] Distance] [Lens] Index] Number]  r1 = −240.881  d1 =  5.00  G1  N1 = 1.67339  ν1 = 29.25  r2 =    48.112  d2 = 10.98  G2  N2 = 1.61800  ν2 = 63.39  r3 =  −51.267  d3 = 18.72  r4 =  −34.071  d4 =  5.00  G3  N3 = 1.67339  ν3 = 29.25  r5 =    49.737  d5 =  1.71  r6 =    69.807  d6 =  8.86  G4  N4 = 1.83350  ν4 = 21.00  r7 =  −42.348  d7 =  0.50  r8 =  −52.992  d8 =  2.00  G5  N5 = 1.67339  ν5 = 29.25  r9 =  −97.001  d9 =  0.50 r10 =    43.391 d10 =  9.07  G6  N6 = 1.61800  ν6 = 63.39 r11 =  −57.547 d11 =  0.50 r12 =  −69.091 d12 =  2.00  G7  N7 = 1.74000  ν7 = 31.72 r13 = −175.888 d13 =  0.50 r14 =    32.474 d14 =  5.27  G8  N8 = 1.78831  ν8 = 47.32 r15 = −340.116 d15 =  2.00  G9  N9 = 1.74000  ν9 = 31.72 r16 =    22.354 d16 = 30.00 r17 = ∞ d17 = 13.00 (Mirror M) r18 = ∞ d18 =  4.50 (Stop S) r19 =    13.417 d19 =  4.00 G10 N10 = 1.83350 ν10 = 21.00 r20 =    12.250 d20 =  5.88 r21 =    15.255 d21 =  3.00 G11 N11 = 1.61800 ν11 = 63.39 r22 = −103.932 d22 =  1.50 r23 =  −27.710 d23 =  4.00 G12 N12 = 1.67339 ν12 = 29.25 r24 =    13.714 d24 =  2.00 r25 =    16.057 d25 =  6.00 G13 N13 = 1.48749 ν13 = 70.44 r26 =    21.020 d26 =  1.00 r27 =   100.722 d27 =  4.00 G14 N14 = 1.61800 ν14 = 63.39 r28 =  −45.399 d28 =  8.06 r29 =    26.041 d29 =  6.00 G15 N15 = 1.83350 ν15 = 21.00 r30 =    36.220

[0069] Embodiment 3 Object distance: 60 mm Effective F. No.: 5.0 Mirror oscillation angle: ±6.1 degrees [Radius of [Axial [Refractive [Abbe Curvature] Distance] [Lens] Index] Number]  r1 =  −202.859  d1 =  5.00  G1  N1 = 1.67339  ν1 = 29.25  r2 =    68.971  d2 = 15.00  G2  N2 = 1.61800  ν2 = 63.39  r3 =  −50.708  d3 = 18.90  r4 =  −38.535  d4 =  5.00  G3  N3 = 1.67339  ν3 = 29.25  r5 =    43.944  d5 = 16.65  G4  N4 = 1.83350  ν4 = 21.00  r6 =  −67.607  d6 =  0.57  r7 =  −87.488  d7 =  5.93  G5  N5 = 1.67339  ν5 = 29.25  r8 =  −193.041  d8 =  0.50  r9 =    57.498  d9 = 10.80  G6  N6 = 1.61800  ν6 = 63.39 r10 =  −43.765 d10 =  2.00  G7  N7 = 1.74000  ν7 = 31.72 r11 =  −473.216 d11 =  0.50 r12 =    28.465 d12 =  7.06  G8  N8 = 1.78831  ν8 = 47.32 r13 =   2190.149 d13 =  2.13  G9  N9 = 1.74000  ν9 = 31.72 r14 =    19.784 d14 = 30.00 r15 = ∞ d15 = 13.00 (Mirror M) r16 = ∞ d16 =  4.50 (Stop S) r17 =    20.435 d17 =  4.00 G10 N10 = 1.83350 ν10 = 21.00 r18 =    28.301 d18 =  4.95 r19 =    23.548 d19 =  3.45 G11 N11 = 1.61800 ν11 = 63.39 r20 =  −36.096 d20 =  1.50 r21 =  −25.425 d21 =  2.00 G12 N12 = 1.67339 ν12 = 29.25 r22 =    13.784 d22 =  1.50 r23 =    14.186 d23 =  6.00 G13 N13 = 1.48749 ν13 = 70.44 r24 =    15.802 d24 = 10.90 r25 =    34.838 d25 =  5.00 G14 N14 = 1.61800 ν14 = 63.39 r26 =  −128.323 d26 =  1.68 r27 =    23.872 d27 =  6.00 G15 N15 = 1.83350 ν15 = 21.00 r28 =    21.263

[0070] Embodiment 4 Object distance: 60 mm Effective F. No.: 5.0 Mirror oscillation angle: ±6.1 degrees [Radius of [Axial [Refractive [Abbe Curvature] Distance] [Lens] Index] Number]  r1 =  −202.555  d1 =  5.00  G1  N1 = 1.67339  ν1 = 29.25  r2 =    80.484  d2 = 15.00  G2  N2 = 1.61800  ν2 = 63.39  r3 =  −44.412  d3 = 10.70  r4 =  −34.397  d4 =  5.00  G3  N3 = 1.74000  ν3 = 31.72  r5 =    47.267  d5 = 20.00  G4  N4 = 4.83350  ν4 = 21.00  r6 =  −111.845  d6 =  0.95  r7 =  −70.466  d7 = 11.00  G5  N5 = 1.61500  ν5 = 63.39  r8 =  −43.286  d8 =  2.00  G6  N6 = 1.74000  ν6 = 31.72  r9 =  −107.883  d9 =  0.50 r10 =  −30.328 d10 =  9.00  G7  N7 = 1.78831  ν7 = 47.32 r11 =    759.423 d11 =  2.00  G8  N8 = 1.74000  ν8 = 31.72 r12 =    20.524 d12 = 30.00 r13 = ∞ d13 = 13.00 (Mirror M) r14 = ∞ d14 =  4.50 (Stop S) r15 =    19.553 d15 =  4.00  G9  N9 = 1.83350  ν9 = 21.00 r16 =    25.954 d16 =  3.49 r17 =    19.469 d17 =  3.00 G10 N10 = 1.61800 ν10 = 63.39 r18 =  −56.217 d18 =  1.50 r19 =  −35.333 d19 =  2.67 G11 N11 = 1.67339 ν11 = 29.25 r20 =    11.612 d20 =  1.50 r21 =    12.549 d21 =  6.00 G12 N12 = 1.48749 ν12 = 70.44 r22 =    14.527 d22 =  6.72 r23 =    39.772 d23 =  3.00 G13 N13 = 1.61800 ν13 = 63.39 r24 =  −180.197 d24 =  6.47 r25 =    18.515 d25 =  3.00 G14 N14 = 1.83350 ν14 = 21.00 r26 =    19.184 Embodiment 4 Object distance: 60 mm Effective F. No.: 5.0 Mirror oscillation angle: ±6.1 degrees [Radius of [Axial [Refractive [Abbe Curvature] Distance] [Lens] Index] Number]  r1 = −4116.921  d1 =  2.00  G1  N1 = 1.83350  ν1 = 21.00  r2 =    30.233  d2 = 11.78  G2  N2 = 1.61800  ν2 = 63.39  r3 =  −128.331  d3 =  7.00  r4 =  −37.813  d4 =  5.00  G3  N3 = 1.74000  ν3 = 31.72  r5 =    90.115  d5 = 12.14  G4  N4 = 1.83350  ν4 = 21.00  r6 =  −54.153  d6 =  0.50  r7 =    48.656  d7 = 15.00  G5  N5 = 1.61800  ν5 = 63.39  r8 =  −64.370  d8 =  3.26  G6  N6 = 1.74000  ν6 = 31.72  r9 =  −218.994  d9 =  0.88 r10 =    25.751 d10 =  8.00  G7  N7 = 1.83350  ν7 = 21.00 r11 =    18.652 d11 = 30.00 r12 = ∞ d12 = 13.00 (Mirror M) r13 = ∞ d13 =  4.50 (Stop S) r14 =    22.510 d14 =  8.00  G8  N8 = 1.61800  ν8 = 63.39 r15 =  −44.563 d15 =  2.61 r16 =  −29.747 d16 =  5.00  G9  N9 = 1.84666  ν9 = 23.82 r17 =    23.336 d17 = 12.33 r18 =    94.078 d18 =  4.00 G10 N10 = 1.83350 ν10 = 21.00 r19 =  −36,841 d19 = 12.75 r20 =    26.392 d20 =  5.42 G11 N11 = 1.61800 ν11 = 63.39 r21 =  −32.560 d21 =  0.60 r22 =  −30.487 d22 =  6.00 G12 N12 = 1.75000 ν12 = 25.14 r23 =    23.340

[0071] FIGS. 11˜15 are aberration diagrams corresponding to the optical systems of examples 1˜5, respectively. In the spherical aberration diagrams, the solid line represents spherical aberration on the C-line, the small dash line represents spherical aberration on the d-line, the dash-dot line represents spherical aberration on the f-line, and the large dash line represents spherical aberration on the g-line. In the astigmatism diagrams, the small dash line S and the large dash line M respectively represent astigmatism on the sagittal plane and meridional plane at the d-line. The distortion diagrams show distortion on the d-line. Conditions (1)˜(6) are satisfied in the optical systems of examples 1˜5. Numerical values of conditions (1)˜(6) in each example 1˜5 are given below. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 |(Σνfp − fm)/Lf| 9.55 4.88 4.88 14.90 2.33 |(Σνrp − rm)/Lr| 35.00 35.00 35.00 35.00 16.47 |φf − 1r/φf| 0.347 0.711 0.721 0.880 0.188 Rf − 1r/Rf − 2f 0.679 1.505 1.316 1.291 3.394 Tf − 12 × φf 0.275 0.224 0.227 0.128 0.084 νrm 29.25 29.25 29.25 29.25 23.82 25.14

[0072] Examples of public disclosure of the previously mentioned conventional art include values of conditions (1)˜(6) listed below in the optical systems of examples 1˜3 disclosed in Japanese Laid-Open Patent No. 9-236741. It will be understood that these values do not necessarily satisfy the conditional range. Ex. 1 Ex. 2 Ex. 3 |(Σνfp − fm)/Lf| 23.90 23.10 28.74 |(Σνrp − rm)/Lr| 17.56 17.38 24.48 |φf − 1r/φf| 0.583 0.988 1.320 Rf − 1r/Rf − 2f 1.111 1.633 −1.340 Tf − 12 × φf −0.583 0.030 0.100 νrm 57.07 57.07 27.51 25.43 25.43 36.66 61.11 61.11

[0073] As described above, the present invention provides a high-performance scanning optical system at low cost, which specifically corrects magnification chromatic aberration and axial chromatic aberration without using a color separation prism, and is not susceptible to performance deterioration when scanning via a mirror.

[0074] The present invention specifically reduces chromatic aberration generated within an object side lens unit, and reduces axial chromatic aberration of the total optical system.

[0075] Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modification will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein. 

What is claimed is:
 1. A scanning optical system comprising: an object side lens unit for condensing light from an object; a mirror for performing a main scan to read the image of the object by deflecting the light transmitted through the object side lens; and an image side lens unit for forming an image on the image sensing surface using both the extra-axial light and axial light in a subscan direction deflected by the mirror; and wherein the following condition is satisfied: 0.1<|(Σνfp−Σνfm)/Lf|<20.0 where Σνfp represents the sum of the Abbe numbers of the positive optical power lenses within the object side lens unit, Σνfm represents the sum of the Abbe numbers of the negative optical power lenses within the object side lens unit, and Lf represents the number of lenses in the object side lens unit.
 2. The scanning optical system according to claim 1, wherein the following condition is satisfied: 10.0<|(Σνrp−Σνrm)/Lr|<50.0 where Σνrp represents the sum of the Abbe numbers of the positive optical power lenses within the image side lens unit, Σνrm represents the sum of the Abbe numbers of the negative optical power lenses within the image side lens unit, and Lr represents the number of lenses in the image side lens unit.
 3. The scanning optical system according to claim 1, wherein the object side lens unit has a cemented lens comprising a negative lens element and a positive lens element on the outermost object side.
 4. The scanning optical system according to claim 3, wherein the cemented lens satisfies the following condition: |φf−1/φf|<1.0 where φf−1 represents the optical power of the cemented lens on the outermost object side within the object side lens unit, and φf represents the optical power of the object side lens unit.
 5. The scanning optical system according to claim 1, wherein the object side lens unit comprises sequentially from the object side an object side front lens unit and an object side back lens unit arranged with a relatively large spacing therebetween.
 6. The scanning optical system according to claim 5, wherein the outermost image side lens element within the object side front lens unit and the outermost object side lens element within the object side back lens unit satisfy the condition below: 0.4<Rf−1r/Rf−2f<5.0 where Rf−1r represents the radius of curvature of the image side surface of the outermost image side lens element within the object side front lens unit, and Rf−2f represents the radius of curvature of the object side surface of the outermost object side lens element within the object side back lens unit.
 7. The scanning optical system according to claim 5, wherein the object side front lens unit satisfies the following condition: 0.05<Tf−12×φf<0.4 where Tf−12 represents the distance between the object side front lens unit and the object side back lens unit, and φf represents the optical power of the object side lens unit. 